The inventive concept relates to an optical communication system and, more particularly, to a resonator including a slab waveguide, a variable wavelength optical filter and a variable wavelength laser diode which have excellent variable wavelength characteristics and support high speed operations.
A variable wavelength optical filter may be realized by various driving methods. A variation speed (or a switching speed) for varying a wavelength and a variable wavelength range are in a trade-off relation in the variable wavelength optical filter. Variable wavelength optical filters in common use have sufficiently wide variable wavelength ranges but provide low switching speeds of, for example, a millisecond level. On the contrary, variable wavelength optical filters having switching speeds of a microsecond level to a nanosecond level have narrow variable wavelength ranges.
A Fabry-Perot (FP) filter corresponding to a typical variable wavelength optical filter has a linear resonator shape including two mirrors. In the FP filter, an incident light may be reflected between the two mirrors or may partially transmit the mirrors, and an interference phenomenon may occur between the reflected and/or transmitted lights. Resonance occurs at regular wavelength intervals by the interference phenomenon, so that the FP filter has a periodic transmission characteristic. For wavelength variation of the FP filter, a distance between the two mirrors may be controlled or refractive indexes of the two mirrors may be controlled.
In the FP filter, the two mirrors may be bonded to a piezoelectric transducer (PZT). A voltage may be applied to the piezoelectric transducer to change the distance between the two mirrors by expansion of the piezoelectric transducer, such that the wavelength variation of the FP filter may be realized. Since the FP filter realizes the wavelength variation by the mechanical change, an operating speed of the FP filter is limited to a millisecond level.
A liquid crystal FP filter may change a refractive index of a liquid crystal disposed within a resonator. The liquid crystal FP shows a Freedericksz transition phenomenon that liquid crystal molecules are rearranged when a predetermined intensity of an electric field is applied to the liquid crystal. Thus, the refractive index of the liquid crystal is changed in the liquid crystal FP filter. The variation speed of the liquid crystal FP filter may be limited by a polarization speed of tens seconds to hundreds seconds.
A micro machined FP filter controls the distance between mirrors by using a semiconductor which is finely modified by electric force or heat. The micro machined FP filter uses a semiconductor device to improve its integration degree and stability. However, the variation speed of the micro machined FP filter may be limited to a speed of tens microseconds to several milliseconds.
A polarization mode conversion filter and a spatial mode conversion filter use mode coupling through energy exchange occurring between modes in an optical waveguide by perturbation. These filter may also have a variation speed of hundreds microseconds or a narrow variable wavelength range.
A filter using a Mach-Zender interferometer has a structure including a phase modulator disposed on an optical waveguide between two couplers. The filter including the Mach-Zender interferometer uses lithium niobate (LiNbO3) as the phase modulator to realize a high speed of several tens nanoseconds. However, the filter using the Mach-Zender interferometer has a complex structure. Thus, it is difficult to fabricate the filter using the Mach-Zender interferometer.
A filter using a diffraction grating changes a period of the grating by lengthening an optical fiber diffraction grating or by applying heat to the grating. However, since the filter of the diffraction grating uses a piezoelectric transducer for a mechanical change of the grating, the filter using the diffraction grating has the variation speed of a millisecond-level.
A distributed feedback (DFB) filter, a distributed Bragg reflector (DBR) filter, and a grating assisted co-directional coupler (GACC) filter having semiconductor waveguide shapes are similar to a semiconductor diode and are controlled by applied currents. The DFB, DBR, and GACC filters realize wavelength variation of several tens nanometers. However, application of the DFB, DBR, and GACC filters may be limited by their narrow variability, discontinuity of variation, and non-stability.
As described above, conventional variable wavelength filters do not satisfy all requirements (i.e., the wide variable wavelength range and the variation speed of a microsecond-level or less).